The present invention relates to a method of making silicone in water emulsions. Specifically, the present invention relates to a simple process of making silicone in water emulsions in which silicones polymerize by chain extension at the interior of a silicone droplet suspended in water.
Several techniques are known in the art for the production of silicone in water emulsions (hereinafter `s/w emulsions`). One technique, emulsion polymerization, is described, for example, in European Patent Applications 268,982, 459,500 and 698,633. This process involves emulsifying a low molecular weight silicone and an anionic or cationic surfactant in a high shear inducing device. The silicone is polymerized by the addition of a strong acid or base, often at elevated temperatures. This process can yield relatively high molecular weight silicone polymer (for example &lt;900,000 mm.sup.2 /sec) in water emulsions.
Emulsion polymerization, however, has a number of drawbacks. For instance, polymerization in the emulsion polymerization process occurs at the silicone water interface. As such, the rate of polymerization is faster with smaller particles because of the larger surface area and, thus, it is impossible to produce large particle size, high molecular weight silicone gum in water emulsions. Similarly, emulsion polymerization involves a number of processing steps and/or materials which are disadvantageous. For instance, emulsion polymerization requires long batch times and caustic materials (strong acidic or basic catalysts which must be neutralized). Finally, the emulsions resulting from emulsion polymerization may have limited utilities because of the materials used in their manufacture. For instance, the anionic and cationic surfactants used in these emulsions can be irritating to the skin and they can affect the stability of products into which the emulsions are incorporated.
A second technique for the production of s/w emulsions is mechanical emulsion. In this process, a silicone polymer is mechanically emulsified with a variety of surfactants and water. This process allows for the production of anionic, cationic, non-ionic or amphoteric emulsions having a variety of particle sizes and high silicone fractions. Similarly, the process is more advantageous than emulsion polymerization in that the processing time is relatively short and the process does not require heating or a neutralization step.
Mechanical emulsion, however, is limited by the shear stress which can be induced by the mixer. For instance, conventional mixers are often limited to silicone polymers with a viscosity of about 600,000 mm.sup.2 /sec at 25.degree. C.
We have now discovered a more desirable method of making s/w emulsions and novel s/w emulsions having a wide range of physical characteristics.